Simulation cell keeps shrinking in fix npt

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1

Dear Lammps users

I am testing a tabular potential for coarse-grained simulation of C70. However, I found that my simulation cell just keeps shrinking when I ran fix npt. My input script is following:

### Test for tabular potential of coarse-grained C70

atom_style full
units real
boundary p p p

C70.table (34.1 KB)

2

Dear Lammps users

I am testing a tabular potential for coarse-grained simulation of C70. However, I found that my simulation cell just keeps shrinking when I ran fix npt. My input script is following:

Setting up run ...
Memory usage per processor = 5.55243 Mbytes
Step Temp E_pair E_mol TotEng Press Volume
       0 300 -1305.5416 0 -412.19284 13.584779 5332375
    1000 241.16809 -1597.9025 0 -879.74511 19.326912 6112178.7
    2000 272.48892 -1256.1123 0 -444.68687 10.655502 6158365.9

[...]

   11000 423.44412 55570.508 0 56831.452 40.648822 115452.91
   12000 387.19636 55898.733 0 57051.738 -254.71216 114884.42
   13000 389.87781 55872.543 0 57033.533 -476.91173 114692.25

It turns out that the system pressure never converged, and the simulation cell keeps shrinking. The simulation eventually crashed shortly after. Could anyone tell me what is the reason leading to this diverging simulation cell in fix npt simulation? The potential table is enclosed

your volume is shrinking by a very large amount in a very short time.
that means the fictitious cell dynamics will pick up a large amount of
momentum in a very short time, and that will result in a significant
compression of your system.

you should use a larger time constant for the box relaxation, or -
even better - additionally prefix the MD with a minimizatiion
including a box relaxation.

axel.

3

Dear Axel

Thank you very much for your prompt reply! I just tried both but it did not work. After a couple thousand fix npt steps same problem occurs. Any ideas?

Cheers

Chun-Wei

4

Dear Axel

Thank you very much for your prompt reply! I just tried both but it did not work. After a couple thousand fix npt steps same problem occurs. Any ideas?

"does not work" is the worst description of a problem. in general, it
is *extremely* difficult to give good advice on extremely minimal or
vague information.

if your simulation keeps crashing, perhaps there is some other reason
that has nothing to do with the cell size or fix npt?

axel.